This invention relates to instrumentation that involves particle counting, and more particularly to a time of flight mass spectrometer capable of handling a large dynamic range of ion fluxes.
Mass spectrometers use the difference in mass-to-charge ratio (m/e) of ionized atoms or molecules to separate them from each other. Mass spectrometry is therefore useful for quantization of atoms or molecules and also for determining chemical and structural information about molecules. Molecules have distinctive fragmentation patterns that provide structural information to identify structural components.
Neutral mass spectrometers must first create gas-phase ions, whereas ion mass spectrometers analyze pre-existing ions. In either case, the ions are then separated in space or time based on their mass-to-charge ratio. Next, the quantity of ions of each mass-to-charge ratio is measured.
In general a mass spectrometer consists of an ionizer (neutral mass spectrometers only), a mass-selective analyzer, and an ion detector. The magnetic-sector, quadrupole, and time-of-flight designs also require extraction and acceleration ion optics to transfer ions from the source region into the mass analyzer.
A time-of-flight (TOF) mass spectrometer uses the differences in transit time through a drift region to separate ions of different masses. Some operate in a pulsed mode so ions must be produced or extracted in pulses, whereas other TOF mass spectrometers measure the times of single ions. An electric field accelerates all ions into a field-free drift region with a kinetic energy of qV, where q is the ion charge and V is the applied voltage. Lighter ions have a higher velocity than heavier ions and reach the detector at the end of the drift region sooner.
One aspect of the invention is a particle detection unit that detects secondary electrons produced in a foil or other emission surface. A detector, such as a microchannel plate detector is used to detect the electrons. A suppression grid is placed in the electron flight path in front of the detector. The grid is made from a conductive material and receives an applied voltage. The applied voltage is set to value that results in a known percentage of the secondary electrons being transmitted through the grid to be detected by the detector.
An advantage of the invention is that it may be used to increase the dynamic range of many types of particle counting instrumentation. More specifically, instruments whose maximum counting rates are limited may be equipped with a particle suppression grid in accordance with the invention, then used for particle fluxes that would otherwise exceed the maximum counting rate.
The invention is especially useful for space applications of time-of-flight mass spectrometers, but may be used for ground spectrometers and other particle counting instrumentation.
In addition to increasing dynamic range, the invention provides a means for maintaining calibration of the counting rate. The same source particles that are analyzed may be used for the calibration, that is, there is no need for any sort of external stimulus or calibration equipment.